1. Field of the Invention
The invention relates to a method and device for jitter enhancement in an optical disc system, more particularly to a method and device for extracting a wobble signal that is used to control rotating speed of a spindle motor from an information signal generated in an optical disc system.
2. Description of the Related Art
In the manufacture of CD-R (Compact Disc-Recordable), an optical disc is formed with a shallow spiral groove, otherwise known as a pre-groove. Due to the presence of the pre-groove, a groove track and a land track are rendered on the surface of the optical disc. Similar to hard disks, the tracks on an optical disc are also divided into sectors. In the case of read-only optical discs, each sector contains time-related information for controlling rotating speed of an optical disc system so that data recorded on the optical disc can be read correctly. For the same reason, there is a need to provide a CD-R with means for guiding advancing movement of laser light emitted from an optical pickup head and for accurately controlling rotating speed in an optical disc system. Therefore, the pre-groove formed on the CD-R is not shaped as a perfect spiral curve, but is somewhat wobbled so as to provide both tracking and timing information. The pre-groove wobbles in a sinusoidal manner such that the optical disc system can generate a wobble signal upon detecting the pre-groove. Based on the frequency and amplitude variation of the wobble signal, the read/write speed of the optical disc system can be adjusted for synchronization with the tracks of the CD-R so as to read/write data correctly.
Since the wobbling structure of a pre-groove is made during the manufacturing process, the structure as such is also known as pre-information. However, as more types of optical discs are being manufactured, the pre-information on an optical disc no longer just includes the aforesaid wobbling structure. For example, in DVD-R/RW (Digital Versatile Disc-Recordable/Rewritable), the pre-information further includes a pre-pit for providing information, such as a read/write address and the optimal power of a laser output for the optical pickup head, etc.
Referring to FIG. 1, a DVD-R 1 is shown to include a protection layer 11, a reflective layer 12, a pigment layer 13, and a substrate layer 14 that protects the pigment layer 13. The pigment layer 13 is formed with a groove track 131 and a land track 132. During the manufacturing process of the DVD-R 1, the groove track 131 is formed as a track wobbled by the frequency corresponding to the rotating speed of the DVD-R 1, whereas the land track 132 is formed with a plurality of pre-pits 15.
As such, when an optical disc system is operated for reading or writing data from/onto the DVD-R 1, a light beam 16 (such as a laser beam) is projected onto the groove track 131 that contains pre-information. Since the diameter of the light beam 16 projected onto the pigment layer 13 is larger than the width of the groove track 131, and since the light beam projected to the reflective layer 12 will be reflected in amounts corresponding to the curvature of the groove track 131, a sinusoidal information signal 17 (as shown in FIG. 2) that corresponds the wobbling structure of the groove track 131 can be obtained. However, due to the effect of the pre-pits 15 on the reflected light beam, noise components (or spikes) 172 corresponding to the pre-pits 15 will be generated within the information signal 17. In other words, the information signal 17 includes a wobble component 171 and a pre-pit component 172. It is noted that only the wobble component 171 is required in the control of the rotating speed of the DVD-R 1 through a spindle motor. Therefore, before the information signal 17 can be sent to a servo circuit that is responsible for controlling the rotation of the spindle motor and the read/write operations, the wobble component 171 must be extracted first (that is, the pre-pit component 172 in the information signal 17 should be removed).
U.S. Pat. No. 6,188,655 discloses a wobble signal extractor for processing the information signal 17 to result in an effective wobble signal that includes the wobble component 171. Referring to FIG. 4, in the wobble signal extractor, a limit level generator 19 and a limiter 20 are used to suppress most of the pre-pit components 171 of the information signal 17 so as to transform the information signal 17 into an effective wobble signal 18, as shown in FIG. 3. A band-pass filter is then used to remove the remaining pre-pit components 182 so that the output signal only contains the wobble component 181, which is subsequently encoded into a digital signal to be fed to the servo circuit for controlling the rotating speed of the spindle motor.
It is noted that the limit level generator 19 in the aforesaid wobble signal extractor includes components, such as a low pass filter 191, an upper side level shift circuit 192 and a lower side level shift circuit 19. Therefore, the limit level generator 19 and the limiter 20 cooperate to form a complicated and large circuit that is costly and difficult to maintain.